WEBVTT 00:00.120 --> 00:00.690 Hello again! 00:01.050 --> 00:04.050 In this video, we are going to look at constant expressions. 00:05.640 --> 00:11.730 A constant expression has a value which is evaluated at compile time, and which cannot be changed. 00:12.300 --> 00:15.420 So obviously, literals would be constant expressions. 00:16.260 --> 00:22.860 We can also have values which are computed entirely from literals. And we can have values which are computed 00:22.860 --> 00:24.540 from other constant expressions. 00:25.140 --> 00:30.870 So if you have a constant expression, it is basically some computation involving literals. 00:33.700 --> 00:37.690 We can also have a variable, which is a constant expression. For this, 00:37.690 --> 00:44.320 the variable has to be initialized from a constant expression, and the variable cannot be modified after 00:44.350 --> 00:45.310 being initialized. 00:46.850 --> 00:51.590 For example, 42 and 99 are literals, so they are constant expressions. 00:51.920 --> 00:57.860 "i" and "j" are initialized from constant expressions, and they cannot be modified after initialization. 00:58.310 --> 01:00.710 So "i" and "j" are constant expressions. 01:01.490 --> 01:06.800 Then "i + j" is a computation involving only constant expressions. 01:07.160 --> 01:10.070 So "i + j" is therefore a constant expression. 01:11.700 --> 01:17.670 So this means you could use "i + j" as the dimension of an array because the dimension of an array must 01:17.670 --> 01:19.050 be a constant expression. 01:21.500 --> 01:27.740 If we make "i" and "j" non-const, then they are still initialized from constant expressions, but 01:27.740 --> 01:30.500 they can now be modified after being initialized. 01:30.890 --> 01:37.730 So "i" and "j" are not constant expressions. And this will give a compiler error. Unless you are using g++, 01:38.180 --> 01:44.300 when you have to use the "-pedantic" option. Because g++ allows this as a non-standard extension. 01:47.010 --> 01:47.940 So here is the code. 01:47.940 --> 01:53.100 We have "i" and "j" initialized from 42 and 99, which are constant expressions. 01:53.910 --> 01:58.950 "i" and "j" cannot be modified after initialization, so they are constant expressions as well. 01:59.520 --> 02:05.190 And then "i" and "j" is a constant expression, and can be used for the dimension of the array. 02:07.480 --> 02:08.470 And then, that compiles. 02:08.710 --> 02:11.470 We just get a warning about the unused local variable. 02:13.670 --> 02:19.280 If I make "i" and "j" non-const, then they are no longer constant expressions. Because they can 02:19.280 --> 02:21.350 be modified after initialization. 02:22.340 --> 02:25.070 And then "i + j" is not a constant expression. 02:25.700 --> 02:29.210 And we get an error. "Did not evaluate to a constant". 02:30.990 --> 02:34.500 So let's look a bit more closely at what it means, for something to be const. 02:35.280 --> 02:38.940 The key word has been around, since the early days of C++. 02:39.960 --> 02:44.310 A variable, which is const, cannot be modified after being initialized. 02:45.180 --> 02:47.370 There are two ways to initialize a const variable. 02:47.970 --> 02:51.240 We can use a constant expression. So we have something like this. 02:51.240 --> 02:53.430 So i is actually a constant expression. 02:54.600 --> 03:00.810 We can also initialize a const variable for a value which is known at runtime. Because const just means you 03:00.810 --> 03:01.710 cannot change the value. 03:02.220 --> 03:07.110 So we could, for example, read something from the keyboard, and then use that to initialize a const 03:07.110 --> 03:07.500 variable. 03:08.760 --> 03:14.190 If we are talking about compile-time programming, which is the subject of this section, then this 03:14.190 --> 03:17.070 could be interesting, because the compiler will know the value of "i". 03:17.490 --> 03:22.800 On the other hand, this is no use. Because compilers have no way of knowing what users are going to do when 03:22.800 --> 03:23.580 the program runs. 03:24.690 --> 03:26.280 So we have two separate concepts here. 03:26.670 --> 03:31.590 And in C++11, we have a new keyword, for the constant expression concept. 03:32.160 --> 03:36.600 So the constexpr keyword means that a variable is a constant expression. 03:37.710 --> 03:42.210 This means that the variable is evaluated at compile time and cannot be modified. 03:42.960 --> 03:48.840 So with the variable "i", which we declared as const before, we can now declare this as constexpr, because 03:48.840 --> 03:50.370 "i" is a constant expression. 03:53.180 --> 03:58.700 So what are the differences between being const and being constexpr? A variable which is const 03:58.700 --> 03:59.930 cannot be modified. 04:00.290 --> 04:02.360 And this is mainly used for function arguments. 04:03.070 --> 04:09.710 We pass an argument by reference to const, or pointer to const, for efficiency and safety. And the compiler 04:09.710 --> 04:13.220 will make sure that the function cannot modify the argument. 04:15.960 --> 04:21.840 If we have a variable which is constexpr, this means the value is known at compile time, and cannot be 04:21.840 --> 04:22.410 modified. 04:23.070 --> 04:26.070 And we use that for computing the value of constants. 04:26.850 --> 04:32.850 And this is useful, because we can improve performance by doing calculations at compile time. Instead 04:32.850 --> 04:33.810 of when the program runs. 04:34.530 --> 04:35.940 Okay, so that s it for this video. 04:36.480 --> 04:37.320 I will see you next time. 04:37.320 --> 04:39.450 But until then, keep coding!